System And Method For Monitoring Compressor Floodback

ABSTRACT

A system and method for a compressor includes a compressor connected to an evaporator, a suction sensor that outputs a suction temperature signal corresponding to a suction temperature of refrigerant entering the compressor, and a control module connected to the suction sensor. The control module determines a saturated evaporator temperature, calculates a suction superheat temperature based on the saturated evaporator temperature and the suction temperature, and monitors a floodback condition of the compressor by comparing the suction superheat temperature with a predetermined threshold. When the suction superheat temperature is less than or equal to the predetermined threshold, the control module increases a speed of the compressor or decreases an opening of an expansion valve associated with the compressor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/247,033 filed on Oct. 7, 2008. This application claims the benefit ofU.S. Provisional Application No. 60/978,258, filed on Oct. 8, 2007 andof U.S. Provisional Application No. 60/978,312, filed Oct. 8, 2007. Theentire disclosures of each of the above applications are incorporatedherein by reference.

FIELD

The present disclosure relates to compressors and more particularly to asystem and method for monitoring a floodback condition of a compressor.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Compressors may be used in a wide variety of industrial and residentialapplications to circulate refrigerant within a refrigeration, heat pump,HVAC, or chiller system (generically “refrigeration systems”) to providea desired heating or cooling effect. In any of the foregoingapplications, the compressor should provide consistent and efficientoperation to insure that the particular application (i.e.,refrigeration, heat pump, HVAC, or chiller system) functions properly. Avariable speed compressor may be used to vary compressor capacityaccording to refrigeration system load. Operating parameters of thecompressor and of the refrigeration system may be used by protection,control, and diagnostic systems to insure optimal operation of thecompressor and refrigeration system components. For example, evaporatortemperature and/or condenser temperature may be used to diagnose,protect, and control the compressor and other refrigeration systemcomponents.

SUMMARY

A system is provided comprising a compressor connected to an evaporator,a suction sensor that outputs a suction temperature signal correspondingto a suction temperature of refrigerant entering the compressor, and acontrol module connected to the suction sensor. The control moduledetermines a saturated evaporator temperature, calculates a suctionsuperheat temperature based on the saturated evaporator temperature andthe suction temperature, and monitors a floodback condition of thecompressor by comparing the suction superheat temperature with apredetermined threshold. When the suction superheat temperature is lessthan or equal to the predetermined threshold, the control moduleincreases a speed of the compressor or decreases an opening of anexpansion valve associated with the compressor.

A method is provided comprising determining, with a control module, asaturated evaporator temperature of an evaporator connected to acompressor and receiving, with the control module, a suction temperaturesignal that corresponds to a suction temperature of refrigerant enteringthe compressor. The method further comprises calculating, with thecontrol module, a suction superheat temperature based on the saturatedevaporator temperature and the suction temperature. The method furthercomprises monitoring, with the control module, a floodback condition ofthe compressor by comparing the suction superheat with a predeterminedthreshold. The method further comprises increasing a speed of thecompressor or decreasing an opening of an expansion valve, with thecontrol module, when the suction superheat temperature is less than orequal to the predetermined threshold.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a schematic view of refrigeration system.

FIG. 2 is a cross-section view of a compressor.

FIG. 3 is a flow chart illustrating steps for an algorithm according thepresent teachings.

FIG. 4 is a graph showing discharge super heat correlated with suctionsuper heat and outdoor temperature.

FIG. 5 is a graph showing discharge line temperature correlated withevaporator temperature and condenser temperature.

FIG. 6 is a graph showing an operating envelope of a compressor.

FIG. 7 is a schematic illustration of another refrigeration system.

FIG. 8 is a graph showing condenser temperature correlated withcompressor power and compressor speed.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

As used herein, the terms module, control module, and controller referto one or more of the following: An application specific integratedcircuit (ASIC), an electronic circuit, a processor (shared, dedicated,or group) and memory that execute one or more software or firmwareprograms, a combinational logic circuit, or other suitable componentsthat provide the described functionality. As used herein, computerreadable medium refers to any medium capable of storing data for acomputer. Computer-readable medium includes, but is not limited to,memory, RAM, ROM, PROM, EPROM, EEPROM, flash memory, CD-ROM, floppydisk, magnetic tape, other magnetic medium, optical medium, or any otherdevice or medium capable of storing data for a computer.

With reference to FIG. 1, an exemplary refrigeration system 5 includes acompressor 10 that compresses refrigerant vapor. While a specificrefrigeration system is shown in FIG. 1, the present teachings areapplicable to any refrigeration system, including heat pump, HVAC, andchiller systems. Refrigerant vapor from compressor 10 is delivered to acondenser 12 where the refrigerant vapor is liquefied at high pressure,thereby rejecting heat to the outside air. The liquid refrigerantexiting condenser 12 is delivered to an evaporator 16 through anexpansion valve 14. Expansion valve 14 may be a mechanical or electronicvalve for controlling super heat of the refrigerant. The refrigerantpasses through expansion valve 14 where a pressure drop causes the highpressure liquid refrigerant to achieve a lower pressure combination ofliquid and vapor. As hot air moves across evaporator 16, the lowpressure liquid turns into gas, thereby removing heat from evaporator16. The low pressure gas is again delivered to compressor 10 where it iscompressed to a high pressure gas, and delivered to condenser 12 tostart the refrigeration cycle again.

Compressor 10 may be monitored and controlled by a control module 25.Control module 25 includes a computer readable medium for storing dataincluding the software executed by a processor to monitor and controlcompressor 10 and to perform the algorithms of the present teachings.

As described in the disclosure titled “VARIABLE SPEED COMPRESSORPROTECTION SYSTEM AND METHOD”, U.S. Application Ser. No. 60/978,258,which is incorporated herein by reference, suction superheat (SSH) maybe used to monitor or predict an overheat condition of compressor 10. Asdescribed therein, an overheat condition is undesirable and may resultin damage to compressor 10, a compressor component, or a refrigerationsystem component.

A compressor floodback or overheat condition is undesirable and maycause damage to compressor 10 or other refrigeration system components.Suction super heat (SSH) and/or discharge super heat (DSH) may becorrelated to a floodback or overheating condition of compressor 10 andmay be monitored to detect and/or predict a floodback or overheatingcondition of compressor 10. DSH is the difference between thetemperature of refrigerant vapor leaving the compressor, referred to asdischarge line temperature (DLT) and the saturated condenser temperature(Tcond). Suction super heat (SSH) is the difference between thetemperature of refrigerant vapor entering the compressor, referred to assuction line temperature (SLT) and saturated evaporator temperature(Tevap).

SSH and DSH may be correlated as shown in FIG. 4. The correlationbetween DSH and SSH may be particularly accurate for scroll typecompressors, with outside ambient temperature being only a secondaryeffect. As shown in FIG. 4, correlations between DSH and SSH are shownfor outdoor temperatures (ODT) of one-hundred fifteen degreesFahrenheit, ninety-five degrees Fahrenheit, seventy-five degreesFahrenheit, and fifty-five degrees Fahrenheit. The correlation shown inFIG. 4 is an example only and specific correlations for specificcompressors may vary by compressor type, model, capacity, etc.

A floodback condition may occur when SSH is approaching zero degrees orwhen DSH is approaching twenty to forty degrees Fahrenheit. For thisreason, DSH may be used to detect the onset of a floodback condition andits severity. When SSH is at zero degrees, SSH may not indicate theseverity of the floodback condition. As the floodback condition becomesmore severe, SSH remains at around zero degrees. When SSH is at zerodegrees, however, DSH may be between twenty and forty degrees Fahrenheitand may more accurately indicate the severity of a floodback condition.When DSH is in the range of thirty degrees Fahrenheit to eighty degreesFahrenheit, compressor 10 may operate within a normal range. When DSH isbelow thirty degrees Fahrenheit, the onset of a floodback condition maybe occur. When DSH is below ten degrees Fahrenheit, a severe floodbackcondition may occur.

With respect to overheating, when SSH is between thirty degreesFahrenheit and fifty degrees Fahrenheit, the onset of an overheatingcondition may occur. When SSH is greater than fifty degrees Fahrenheitor when DSH is greater than one-hundred degrees Fahrenheit, a severeoverheating condition may be present.

In FIG. 4, typical SSH temperatures for exemplar refrigerant chargelevels are shown. For example, as the percentage of refrigerant chargein refrigeration system 5 decreases, SSH typically increases.

With reference to FIG. 1, evaporator 16 may include an evaporatortemperature sensor 40 that may sense an evaporator temperature.Alternatively, an evaporator pressure sensor may be used. Control module25 receives evaporating temperature (Tevap) from evaporator temperaturesensor 40.

A suction sensor 34 monitors a temperature of refrigerant enteringcompressor 10 (i.e., SLT). Alternatively, a combination suctiontemperature/pressure sensor may be used. In such case, control module 25may receive SLT from the temperature portion of the sensor and Tevapfrom the pressure portion of the sensor, as Tevap may be derived ormeasured based on suction pressure. Further, Tevap may be derived fromother system parameters, as disclosed in the disclosure titled “VARIABLESPEED COMPRESSOR PROTECTION SYSTEM AND METHOD”, U.S. Application Ser.No. 60/978,258, which is incorporated herein by reference.

For example, Tevap may be derived as a function of Tcond and DLT, asdescribed in commonly assigned U.S. application Ser. No. 11/059,646,U.S. Publication No. 2005/0235660. For variable speed compressors, thecorrelation may also reflect compressor speed. In this way, Tevap may bederived as a function of Tcond, DLT and compressor speed.

As shown in FIG. 5, Tevap is shown correlated with DLT, for variousTcond levels. For this reason, compressor map data for different speedsmay be used.

Tcond and Tevap may be calculated based on a single derivation.

In addition, iterative calculations may be made based on the followingequations:

Tcond=f(compressor power,compressor speed,Tevap)  Equation 1:

Tevap=f(Tcond,DLT,compressor speed)  Equation 2:

Multiple iterations of these equations may be performed to achieveconvergence. For example, three iterations may provide optimalconvergence. As discussed above, more or less iteration, or noiterations, may be used.

Tevap and Tcond may also be determined by using compressor map data, fordifferent speeds, based on DLT and compressor power, based on thefollowing equations:

Tevap=f(compressor power,compressor speed,DLT)  Equation 3:

Tcond=f(compressor power,compressor speed,DLT)  Equation 4:

Once Tevap and Tcond are known, additional compressor performanceparameters may be derived. For example, compressor capacity andcompressor efficiency may be derived based on additional compressorperformance map data for a specific compressor model and capacity. Suchadditional compressor map data may be derived from test data. Forexample, compressor mass flow or capacity, may be derived according tothe following equation:

Tevap=f(compressor speed,Tcond,mass flow)  Equation 5:

Control module 25 may calculate Tevap or receive Tevap data from thepressure portion of sensor 34. Control module 25 may then calculate SSHas a difference between SLT and Tevap.

As shown in FIG. 1, suction sensor 34 is external to compressor 10 andmonitors a temperature of refrigerant as it is entering the suctioninlet of compressor 10. Alternatively, a suction sensor internal to thecompressor may be used. As shown in FIG. 2, a suction sensor 32 may bedisposed within a shell of compressor 10. In such case, SLT may becommunicated to control module 25 through an electrical connection viaterminal box 24.

Control module 25 may monitor an overheat condition of compressor 10 bycomparing SSH with a predetermined overheat threshold. As shown in FIG.3, control module 25 receives SLT data in step 302. In step 304, controlmodule 25 receives Tevap from evaporator temperature sensor 40. In step306, control module 25 calculates SSH based on SLT and Tevap.Alternatively, Tevap may be estimated or derived based on other sensedparameters, as described above and in the disclosure titled “VARIABLESPEED COMPRESSOR PROTECTION SYSTEM AND METHOD”, U.S. Application Ser.No. 60/978,258, which is incorporated herein by reference.

In step 308, control module compares SSH with a predetermined thresholdto determine whether an overheat condition exists.

Control module 25 may determine that compressor 10 is operating within anormal temperature range when SSH is between zero and thirty degreesFahrenheit. When SSH is between thirty degrees Fahrenheit and fiftydegrees Fahrenheit, control module 25 may detect an overheat conditionand take responsive measures. A SSH temperature above fifty degreesFahrenheit may indicate that components of the compressor, including thecompressor scrolls, bearings, etc., are at risk of being damaged.

Control module 25 may also determine whether SSH is greater than apredetermined threshold for a predetermined period of time. For example,control module 25 may determine when SSH is between thirty degrees andfifty degrees Fahrenheit, or greater than fifty degrees Fahrenheit, fora predetermined period. For example, the predetermined period may be anumber of minutes (e.g., one minute, two minutes, five minutes, etc.). Afirst predetermined period (e.g., five minutes) may be used formonitoring when SSH is between thirty degrees and fifty degreesFahrenheit. A second predetermined period, shorter than the firstpredetermined period, (e.g., one minute or two minutes) may be used formonitoring when SSH is greater than fifty degrees Fahrenheit. It isunderstood that any time period may be used as appropriate.

As described in the disclosure titled “VARIABLE SPEED COMPRESSORPROTECTION SYSTEM AND METHOD”, U.S. Application Ser. No. 60/978,258,which is incorporated herein by reference, in response to an overheatcondition, control module 25 may adjust compressor operation and/oradjust expansion valve 14. In a severe overheat condition, controlmodule 25 may stop operation of compressor 10. Control module 25 mayalso generate an alarm or notification that an overheat conditionexists.

As shown in FIG. 6, a compressor operating envelope may provide maximumfloodback and maximum SSH limits. In addition, a maximum scrolltemperature limit (Tscroll) may be provided, in the case of a scrollcompressor. In addition, a maximum motor temperature (Tmotor) may beprovided. As shown in FIG. 6, compressor speed and expansion valve 14may be adjusted based on SSH to insure compressor operation within thecompressor operating envelope. In this way, SSH may be maintained withinan acceptable range as indicated by FIG. 6. Compressor speed adjustmentmay take priority over expansion valve adjustment. In some cases, suchas a defrost state, it may be difficult for expansion valve 14 torespond quickly and compressor speed adjustment may be more appropriate.

For example, at a SSH between thirty degrees Fahrenheit and fiftydegrees Fahrenheit, control module 25 may reduce compressor speed orcause expansion valve 14 to open. At a SSH greater than fifty degreesFahrenheit, control module 25 may stop operation of compressor 25.

With reference to FIG. 7, another exemplary refrigeration system 5 aincludes a compressor 10 that compresses refrigerant vapor. While aspecific refrigeration system is shown in FIG. 7, the present teachingsare applicable to any refrigeration system, including heat pump, HVAC,and chiller systems. Refrigerant vapor from compressor 10 is deliveredto a condenser 12 where the refrigerant vapor is liquefied at highpressure, thereby rejecting heat to the outside air. The liquidrefrigerant exiting condenser 12 is delivered to an evaporator 16through an expansion valve 14. Expansion valve 14 may be a mechanical orelectronic valve for controlling super heat of the refrigerant. Therefrigerant passes through expansion valve 14 where a pressure dropcauses the high pressure liquid refrigerant to achieve a lower pressurecombination of liquid and vapor. As hot air moves across evaporator 16,the low pressure liquid turns into gas, thereby removing heat fromevaporator 16. The low pressure gas is again delivered to compressor 10where it is compressed to a high pressure gas, and delivered tocondenser 12 to start the refrigeration cycle again.

With reference to FIG. 7, compressor 10 may be driven by an inverterdrive 22, also referred to as a variable frequency drive (VFD), housedin an enclosure 20. Enclosure 20 may be near compressor 10. Inverterdrive 22 receives electrical power from a power supply 18 and deliverselectrical power to compressor 10. Inverter drive 22 includes a controlmodule 25 with a processor and software operable to modulate and controlthe frequency of electrical power delivered to an electric motor ofcompressor 10. Control module 25 includes a computer readable medium forstoring data including the software executed by the processor tomodulate and control the frequency of electrical power delivered to theelectric motor of compressor and the software necessary for controlmodule 25 to execute and perform the protection and control algorithmsof the present teachings. By modulating the frequency of electricalpower delivered to the electric motor of compressor 10, control module25 may thereby modulate and control the speed, and consequently thecapacity, of compressor 10.

Inverter drive 22 includes solid state electronics to modulate thefrequency of electrical power. Generally, inverter drive 22 converts theinputted electrical power from AC to DC, and then converts theelectrical power from DC back to AC at a desired frequency. For example,inverter drive 22 may directly rectify electrical power with a full-waverectifier bridge. Inverter driver 22 may then chop the electrical powerusing insulated gate bipolar transistors (IGBT's) or thyristors toachieve the desired frequency. Other suitable electronic components maybe used to modulate the frequency of electrical power from power supply18.

Electric motor speed of compressor 10 is controlled by the frequency ofelectrical power received from inverter driver 22. For example, whencompressor 10 is driven at sixty hertz electric power, compressor 10 mayoperate at full capacity operation. When compressor 10 is driven atthirty hertz electric power, compressor 10 may operate at half capacityoperation.

Piping from evaporator 16 to compressor 10 may be routed throughenclosure 20 to cool the electronic components of inverter drive 22within enclosure 20. Enclosure 20 may include a cold plate 15. Suctiongas refrigerant may cool the cold plate prior to entering compressor 10and thereby cool the electrical components of inverter drive 22. In thisway, cold plate 15 may function as a heat exchanger between suction gasand inverter drive 22 such that heat from inverter drive 22 istransferred to suction gas prior to the suction gas entering compressor10.

To determine DSH, DLT may be subtracted from Tcond. DLT may be sensed bya DLT sensor 28 that senses a temperature of refrigerant exitingcompressor 10. As shown in FIG. 7, DLT sensor 28 may be external tocompressor 10 and may be mounted proximate a discharge outlet ofcompressor 10. Alternatively, an internal DLT sensor may be used. Theinternal DLT sensor may be embedded in an upper fixed scroll of a scrollcompressor and may sense a temperature of discharge refrigerant exitingthe intermeshing scrolls.

In the alternative, a combination temperature/pressure sensor may beused. In such case, Tcond may be measured based on the pressure ofrefrigerant exiting compressor 10 as measured by the combination sensor.Moreover, in such case, DSH may be calculated based on DLT, as measuredby the temperature portion of the sensor, and on Tcond, as measured bythe pressure portion of the combination sensor.

Tcond may be derived from other system parameters. Specifically, Tcondmay be derived from compressor current and voltage (i.e., compressorpower), compressor speed, and compressor map data associated withcompressor 10. A method for deriving Tcond based on current, voltage andcompressor map data for a fixed speed compressor is described in thecommonly assigned application for Compressor Diagnostic and ProtectionSystem, U.S. application Ser. No. 11/059,646, Publication No. U.S.2005/0235660. Compressor map data for a fixed speed compressorcorrelating compressor current and voltage to Tcond may be compressorspecific and based on test data for a specific compressor type, modeland capacity.

In the case of a variable speed compressor, Tcond may also be a functionof compressor speed, in addition to compressor power.

A graphical correlation between compressor power in watts and compressorspeed is shown in FIG. 8. As shown, Tcond is a function of compressorpower and compressor speed. In this way, a three-dimensional compressormap with data correlating compressor power, compressor speed, and Tcondmay be derived for a specific compressor based on test data. Compressorcurrent may be used instead of compressor power. Compressor power,however, may be preferred over compressor current to reduce the impactof any line voltage variation.

In this way, control module 25 may calculate Tcond based on compressorpower data and compressor speed data. Control module 25 may calculate,monitor, or detect compressor power data during the calculationsperformed to convert electrical power from power supply 18 to electricalpower at a desired frequency. In this way, compressor power and currentdata may be readily available to control module 25. In addition, controlmodule 25 may calculate, monitor, or detect compressor speed based onthe frequency of electrical power delivered to the electric motor ofcompressor 10. In this way, compressor speed data may also be readilyavailable to control module 25. Based on compressor power and compressorspeed, control module 25 may derive Tcond.

After measuring or calculating Tcond, control module 25 may calculateDSH as the difference between Tcond and DLT, with DLT data beingreceived from external DLT sensor 28 or an internal DLT sensor.

Control module 25 may monitor DSH to detect a floodback or overheatcondition, based on the correlation between DSH and floodback andoverheat conditions described above. Upon detection of a floodback oroverheat condition, control module 25 may adjust compressor speed oradjust expansion valve 14 accordingly. Control module 25 may communicatewith or control expansion valve 14. Alternatively, control module 25 maycommunicate with a system controller for refrigeration system 5 and maynotify system controller of the floodback or overheat condition. Systemcontroller may then adjust expansion valve or compressor speedaccordingly.

DSH may be monitored to detect or predict a sudden floodback or overheatcondition. A sudden reduction in DLT or DSH without significantaccompanying change in Tcond may be indicative of a sudden floodback oroverheat condition. For example, if DLT or DSH decreases by fiftydegrees Fahrenheit in fifty seconds, a sudden floodback condition mayexist. Such a condition may be caused by expansion valve 14 being stuckopen. Likewise, a sudden increase in DLT or DSH with similar magnitudeand without significant accompanying change in Tcond may be indicativeof a sudden overheat condition due to expansion valve 14 being stuckclosed.

In the event of a floodback condition, control module 25 may limit acompressor speed range. For example, when DSH is below thirty degreesFahrenheit, compressor operation may be limited to the compressor'scooling capacity rating speed. For example, the cooling capacity ratingspeed may be 4500 RPM. When DSH is between thirty degrees Fahrenheit andsixty degrees Fahrenheit, compressor operating speed range may beexpanded linearly to the full operating speed range. For example,compressor operating speed range may be between 1800 and 7000 RPM.

What is claimed is:
 1. A system comprising: a compressor connected to anevaporator; a suction sensor that outputs a suction temperature signalcorresponding to a suction temperature of refrigerant entering thecompressor; a control module connected to the suction sensor, saidcontrol module determining a saturated evaporator temperature,calculating a suction superheat temperature based on the saturatedevaporator temperature and the suction temperature, monitoring afloodback condition of the compressor by comparing the suction superheattemperature with a predetermined threshold, and, when the suctionsuperheat temperature is less than or equal to the predeterminedthreshold, increasing a speed of the compressor or decreasing an openingof an expansion valve associated with the compressor.
 2. The system ofclaim 1 wherein the predetermined threshold is zero degrees Fahrenheit.3. The system of claim 1 wherein the control module increases the speedof the compressor when the suction superheat temperature is less than orequal to the predetermined threshold.
 4. The system of claim 1 whereinthe control module decreases the opening of the expansion valve when thesuction superheat temperature is less than or equal to the predeterminedthreshold.
 5. The system of claim 1 wherein the control module receivesa suction pressure signal corresponding to a suction pressure ofrefrigerant entering the compressor and determines the saturatedevaporator temperature based on the suction pressure.
 6. The system ofclaim 1 further comprising a condenser connected to the compressor andthe evaporator, wherein the control module determines a discharge linetemperature corresponding to a temperature of refrigerant leaving thecompressor, determines a saturated condenser temperature, and determinesthe saturated evaporator temperature as a function of the saturatedcondenser temperature, the discharge line temperature, and the speed ofthe compressor.
 7. The system of claim 6 wherein the control modulereceives compressor power data and determines the saturated condensertemperature as a function of the compressor power data, the speed of thecompressor, and the saturated evaporator temperature.
 8. The system ofclaim 7 wherein the control module performs multiple iterations ofdetermining the saturated condenser temperature and the saturatedevaporator temperature to achieve convergence.
 9. The system of claim 1wherein the control module determines a discharge line temperaturecorresponding to a temperature of refrigerant leaving the compressor,receives compressor power data, and determines the saturated evaporatortemperature as a function of the compressor power data, the speed of thecompressor, and the discharge line temperature.
 10. The system of claim1 further comprising a condenser connected to the compressor and theevaporator, wherein the control module determines a saturated condensertemperature, determines a mass flow of the compressor, and determinesthe saturated evaporator temperature as a function of the speed of thecompressor, the saturated condenser temperature, and the mass flow. 11.A method comprising: determining, with a control module, a saturatedevaporator temperature of an evaporator connected to a compressor;receiving, with the control module, a suction temperature signal thatcorresponds to a suction temperature of refrigerant entering thecompressor; calculating, with the control module, a suction superheattemperature based on the saturated evaporator temperature and thesuction temperature; monitoring, with the control module, a floodbackcondition of the compressor by comparing the suction superheat with apredetermined threshold; and increasing a speed of the compressor ordecreasing an opening of an expansion valve, with the control module,when the suction superheat temperature is less than or equal to thepredetermined threshold.
 12. The method of claim 11 wherein thepredetermined threshold is zero degrees Fahrenheit.
 13. The method ofclaim 11 wherein the control module increases the speed of thecompressor when the suction superheat temperature is less than or equalto the predetermined threshold.
 14. The method of claim 11 wherein thecontrol module decreases the opening of the expansion valve when thesuction superheat temperature is less than or equal to the predeterminedthreshold.
 15. The method of claim 11 further comprising receiving, withthe control module, a suction pressure signal corresponding to a suctionpressure of refrigerant entering the compressor and determining thesaturated evaporator temperature based on the suction pressure.
 16. Themethod of claim 11 further comprising determining, with the controlmodule, a discharge line temperature corresponding to a temperature ofrefrigerant leaving the compressor, determining a saturated condensertemperature of a condenser connected to the evaporator and thecompressor, and determining the saturated evaporator temperature as afunction of the saturated condenser temperature, the discharge linetemperature, and the speed of the compressor.
 17. The method of claim 16further comprising receiving, with the control module, compressor powerdata and determining the saturated condenser temperature as a functionof the compressor power data, the speed of the compressor, and thesaturated evaporator temperature.
 18. The method of claim 17 furthercomprising performing multiple iterations of determining the saturatedcondenser temperature and determining the saturated evaporatortemperature to achieve convergence.
 19. The method of claim 11 furthercomprising determining, with the control module, a discharge linetemperature corresponding to a temperature of refrigerant leaving thecompressor, receiving compressor power data, and determining thesaturated evaporator temperature as a function of the compressor powerdata, the speed of the compressor, and the discharge line temperature.20. The method of claim 11 further comprising determining, with thecontrol module, a saturated condenser temperature of a condenserconnected to the evaporator and the compressor, determining a mass flowof the compressor, and determining the saturated evaporator temperatureas a function of the speed of the compressor, the saturated condensertemperature, and the mass flow.